Coherent optical techniques such as holography, interferometry, electronic speckle pattern interferometry (ESPI), speckle interferometry, particle image velocimetry (PIV) and shearography are currently being utilised for applications such as non-destructive testing (NDT), vibration analysis, object contouring, stress and strain measurement, fatigue testing, deformation analysis and fluid flow diagnosis. All these techniques have associated drawbacks with performance being to some extent a trade off against specific disadvantages inherent in the individual techniques.
For example shearography has high sensitivity and tolerance to environmental noise but is of limited application because of difficulties in inspecting large areas due to inefficiencies in the laser power available and optical beam expansion and delivery systems. Additional problems are encountered with a relatively low signal to noise ratio.
Coherent optical techniques produce an image of the test piece sample that is overlaid with interference fringes (bands of high and low intensity). Depending on which technique is being applied, these fringes denote, for example, loci of displacement, strain, air density. The fringes are produced by subtracting speckle images, or reconstructed wavefronts, of a sample that has undergone some form of deformation between the two exposures. With speckle techniques the subtraction is usually performed electronically using a Charge Coupled Device (CCD) camera, framestore and computer, whereas with holographic interferometry the subtraction occurs when two reconstructed wavefronts (or one actual scattered wavefront and one reconstructed wavefront) are made to interfere. The resulting fringe pattern contours this deformation.
In practice, it is difficult to extract quantitative physical information from fringe patterns. Direct data extraction requires automated fringe tracking and interpolation. This process is fraught with problems due to variations in illumination and object reflectance. In addition, with infinite fringe patterns, that is fringes visible when the sample is undistorted, it is impossible to determine whether the phase is increasing or decreasing between adjacent fringes. Two methods have been proposed to remove this ambiguity and offer varying degrees of noise tolerances.
The first known phase extraction technique known as phase stepping involves capturing a sequence of separate interferometric images, as the phase of one of the interfering wavefronts is incremented. Commonly three images are recorded that are each stepped in phase by 2.pi./3. Mathematically, the intensity distribution of these temporally separated fringe patterns (in this example for three steps) may be donor bed by equation (1). EQU i.sub.n (x,y)=a(x,y)+b(x,y)cos[.phi.(x,y)+2.pi..OMEGA./3] (1)
Where,
i.sub.n (x,y)--nth recorded interferogram PA1 a (x,y)--unmodulated background intensity PA1 b (x,y)--fringe modulation depth PA1 .phi. (x,y)--phase term describing the phase encoded in the fringes PA1 n--integer denoting the number of phase steps Subsequently, the phase data at each pixel .phi. (x,y) may be extracted by applying equation 2. ##EQU1##
In general, phase stepping is good at removing speckle noise and producing clean images from interferograms that have poor contrast. It is the preferred conventional method, with the exception that the three images have to be acquired sequentially over a period that is usually at least three frames times, that is 120 ms before and after object deformation. In practice, this duration is too long to study many important dynamic events and prevents the use of continuous-wave phase stepped interferometry in industrial environments with the exception of shearography in certain circumstances. Also, the need for multiple image captures prevents real-time phase map display unless multiple cameras are used which is impracticable.
The second known method involves superimposing the phase data on a spatial carrier wave, by changing the angle between the interfering wavefronts for the second exposure. Physically this produces a set of closely spaced parallel, that is, finite, fringes, that are specially perturbed by the phase data related to the object deformation. The required phase date is extracted by filtering the unmodulated information and demodulating the phase data by Fourier transform techniques.
The technique that superimposes the data on a spatial carrier is more versatile than phase stepping, because the interferometric phase date may be extracted from a single image. Consequently, if the angle between the interfering wavefronts can be changed between the image captures, either optically or digitally, in its undeformed and deformed conditions, then dynamic events may be studied. However, this usually requires the use of large, expensive and unreliable high power pulsed lasers and electro-optic beam switching of the reference beams. Also, the technique does not work well with noisy speckle images and the gross spatial filtering that is needed to clean up the images results in corruption of the required phase data.